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Lecture Notes: Avian Evolution

Class Business
·  Did you meet Dr. Rubega yet? No? You are late! Run her down, and sit down for a five minute chat. It’s easy!

·  Do we have your Course Guidelines sheet? If not, you are NOT getting any of the available points! Do we have your Twitter Handle Worksheet? If not, then you are NOT getting points for your tweets! Now we will cease to remind you.

·  The class standard for naming groups of birds = that presented on the Tree of Life Project web page for Neornithes. Link is on the class web page.

· Note that there are links on the class web page to the newest scientific papers on the phylogeny of birds. These are likely to contain news you should consider in your Ordinal summary.


Reading for this lecture
Required:  Gill: Chapter 2.

Optional: Proctor & Lynch: Pages 13-21.

1. Origins of Birds

A)    How do we determine how birds have evolved?

i)               Ideas about the evolution of birds are HYPOTHESES

a.      A hypothesis is a FORMAL guess about the how the world works

i.     You have to say specifically what your guess is

ii.     You have to say what the evidence will look like if your guess is right

iii.     You have to say what the evidence will look like if your guess is WRONG

iv.     Any guess for which you can’t do (iii) is not a hypothesis!

v.     We test hypotheses by looking for evidence, especially type (iii) evidence

B)    How do we know that birds evolved from reptiles?

i)      Birds share many features in common with modern reptiles, and differ in significant ways from mammals.  These features include: a lower jaw made up of several bones fused together (mammals have 1 bone), a single bone in the middle ear (mammals have 3), a single occipital condyle joining the skull to the first neck vertebra (mammals have 2), a sclerotic eye ring, similar scales on legs to reptile scales, nucleated red blood cells (in mammals, red blood cells lack nuclei), similarities in their eggs compared to reptiles, etc.

ii)     This relationship has been known since the 1860s, when Sir Thomas Huxley described birds as “glorified reptiles”.

iii)    Although it is well accepted that birds are reptiles, there remains a (rapidly fading) debate about which group of reptiles birds evolved from.  There is now broad consensus, based on fossil evidence (see below) however, that birds arose from therapod dinosaurs.

C)    Archaeopteryx lithographica

i)      First fossil found in 1861 in Germany.  Crow-sized.  Probably terrestrial, but capable of climbing and weak flight.  Alive in late Jurassic (135-155 million years ago).

ii)     “Missing link” between reptiles and birds.  Shares features of bird: looks like a therapod (specifically, a coelurosaur) dinosaur, but it has feathers that are essentially identical to those of modern birds.

iii)   First specimen was identified shortly after Darwin published the “Origin of Species”.  Soon became a strong piece of evidence for the idea that different types of organisms are related to one another and have common ancestors.

D)    Which reptiles did birds arise from?

i)      Most people believe that it was the theropod dinosaurs.  Archaeopteryx and modern birds both share many features with theropods.

ii)     Key similarities that birds share with theropods include: features of the skull (fenestra, sclerotic ring); a long, flexible, S-shaped neck; fused clavicles (“collarbones”; forming the furcula, or “wishbone”), sternum (breastbone), and pelvic bones; elongation of the digits in the forelimbs; bipedalism; digitigrade walkers (walk on toes); pneumatic (i.e., filled with air spaces) bones.

iii)   Differences between modern birds and Archaeopteryx include: toothless beak; fused “hand” elements; tail bones greatly reduced to form the pygostyle; deeply keeled sternum (this last thing is absent in flightless birds).

iv)    Alternative idea:  A few paleontologists do not believe the theropod origin of birds.  Instead they argue that birds evolved from an earlier group of reptiles, the thecodonts.

E)     Recent discoveries

i)      In recent years new fossils have been found, many of them in China, which have strengthened the case for a link between birds and dinosaurs.  These include fossils of animals that are clearly therapod dinosaurs, but which have feathers or feather-like structures (e.g., Sinosauropteryx, Caudipteryx), and also fossils of animals that appear to be intermediate between Archaeopteryx and modern birds (e.g., Sinornis santensis).

F)     Evolution of modern birds

i)               During the Tertiary period there was a great deal of diversification of birds.

ii)              Evolution of some truly enormous birds, including predators 2-3 m tall (e.g., Diatrymas) and vulture-like birds with up to 8 m wingspans (Teratorns).

iii)            Orders of modern birds arose about 60 million years ago, early in the Tertiary.

iv)             Many modern genera of birds were present by the end of the Tertiary (5-10 million years ago).